This invention relates generally to the field of integrated optics chips or devices and more particularly to the field of multifunction integrated optics chips such as those having integrated optic components formed in lithium niobate (LiNbO3) substrates. Integrated optics components formed on such chips include waveguides that may be arranged to function as optical couplers, polarizers and phase modulators. Multiple functions may be incorporated on a single device, which eliminates losses and errors associated with interfacing separate devices. This invention is particularly directed to methods and apparatus for improving polarization extinction ratio or for reducing polarization non-reciprocity (PNR) errors in a multifunctional integrated optics chip as a result of surface wave propagations, which can cross couple into optical fibers through the fiber optic pigtails that are connected to the optical waveguides.
Multifunctional Integrated Optical Chips (MIOC""s) are usually fabricated in large numbers on three to four inch wafers of lithium niobate (LiNbO3) using conventional photomasks, vacuum deposition, chemical baths, proton exchange, diffusion and etching techniques to form large numbers of identical components at low cost and with high reliability. MIOC""s capable of performing many of the aforementioned functions are used in fabricating middle and high accuracy fiber optic gyroscopes (FOG""s) or rotation sensors. The FOG uses the Sagnac effect to measure rates of rotation about an axis perpendicular to a coil of optical fiber. MIOC""s may also be used in forming other fiber optic sensors such as hydrophones or geophones that rely on the principles of the Mach-Zehnder or Michelson interferometer.
A fiber optic gyroscope includes means for introducing counterpropagating waves from an optical signal source into an optical fiber coil. Rotation of the coil about an axis perpendicular to the plane of the coil produces a phase difference between the clockwise and counterclockwise waves via the Sagnac effect. The phase shift occurs because waves that traverse the coil in the direction of the rotation have a longer transit time through the optical fiber coil than waves that traverse the coil in the opposite direction. The waves are combined after propagating through the coil. This combination of waves produces an interference pattern that may be processed to determine the rate of rotation. Techniques for determining the rotation rate are well-known in the art.
It is common practice to form a FOG to include a multifunctional integrated optics chip (MIOC) between the optical signal source and the optical fiber coil, which is typically formed of polarization maintaining fiber. The MIOC typically includes a plurality of optical waveguides arranged to form a Y-junction. The base of the Y-junction is connected to the optical signal source while the arms of the Y-junction are interfaced with ends of the optical fiber coil. Optical signals input to the multifunctional integrated optics chip divide at the Y-junction to form optical signals that are input to the ends of the optical fiber coil as the counterpropagating waves. After propagating through the coil, the waves enter the optical waveguides that form the arms of the Y-junction. The waves then combine in the Y-junction and are output from the base of the Y-junction to an optical fiber. The combined waves are guided to a photodetector that produces an electrical signal that is processed to determine the rotation rate.
The desired condition in a fiber optic rotation sensor is the transverse electric (TE) mode propagating in the optical fiber coil and in the optical waveguides without added path lengths. Propagation of TE modes having added path lengths and transverse magnetic (TM) modes are undesired conditions. Error sources such as polarization cross coupling, which adds a phase shift (or polarization non-reciprocity, PNR, which is associated with always having two polarization components possible in the fiber at all times), manifest themselves as additional optical path differences in direct competition with the Sagnac effect. These error sources cause phase bias and amplitude bias errors when they are modulated at the frequency used by the phase modulators in the MIOC. The bias component in the fiber optic rotation sensor due to polarization cross coupling is inversely proportional to the square root of the absolute value of the polarization extinction ratio. Extinction ratio is defined as ten times the logarithm of the ratio of the undesired power (the power of the undesired mode) to the desired power (the power of the desired mode) of the polarization modes expressed in decibels. Minimizing cross coupling (maximizing the absolute value of the extinction ratio) in the MIOC reduces this type of bias error.
As further background, integrated optics chips (IOCs), such as those disclosed herein may be formed using processes and steps similar to some of those disclosed in U.S. Pat. No. 5,193,136, which issued to Chin L. Chang et al. on Mar. 9, 1993 for PROCESS FOR MAKING MULTIFUNCTION INTEGRATED OPTICS CHIPS HAVING HIGH ELECTRO-OPTIC COEFFICIENTS; U.S. Pat. No. 5,046,808, which issued to Chin L. Chang on Sep. 10, 1991 for INTEGRATED OPTICS CHIP AND METHOD OF CONNECTING OPTICAL FIBER THERETO; U.S. Pat. No. 5,393,371, which issued to Chin L. Chang et al. on Feb. 28, 1995 for INTEGRATED OPTICS CHIPS AND LASER ABLATION METHODS FOR ATTACHMENT OF OPTICAL FIBERS THERETO FOR LiNbO3 SUBSTRATES; U.S. Pat. No. 5,442,719, which issued to Chin L. Chang et al. on Aug. 15, 1995 for ELECTRO-OPTIC WAVEGUIDES AND PHASE MODULATORS AND METHODS FOR MAKING THEM; and U.S. Pat. No. 4,976,506, which issued to George A. Pavlath on Dec. 11, 1990 for METHODS FOR RUGGED ATTACHMENT OF FIBERS TO INTEGRATED OPTICS CHIPS AND PRODUCT THEREOF.
Each of the foregoing patents is assigned to Litton Systems, Inc. of Woodland Hills, Calif. Each of the foregoing patents cited above is incorporated herein by reference for the purpose of providing those skilled in the art with background information on how integrated optics chips or multifunctional integrated optics chips are made.
Conventional MIOC""s do not use any known method of reducing or eliminating surface propagating light rays, which can cross couple into the opposite pigtail(s). These weakly guided rays may be propagated by a thin (less than 1 micron) slab waveguide on the top surface of the MIOC. It has been found that during high temperature processes often used in fabricating the IOC, lithium ions out-diffuse from the crystal surface, forming a high-index layer which acts as a surface slab waveguide. Leakage may occur from the channel waveguide into this slab. Weakly guided surface modes also have the potential of traveling along the surface towards the sides, reflecting off the top edge and back to be coupled into a pigtail. However, this is not as likely because the top edge of the chip is more likely to reflect this light down into the chip unless it has a perfectly square top edge.
If the gyro bias is significantly reduced, there is the potential to reduce the fiber costs by replacing polarization maintaining fiber with less expensive single mode fiber, or by using a shorter length of polarization maintaining fiber than is presently used. There is also the potential to support increased gyro accuracy.
The present invention is designed to extinguish, divert, or trap the various light paths that could potentially cross couple through reflections off the top and sides of an integrated optics chip. The present invention incorporates a passive method for manipulation of a surface propagating wave by diverting it to a position where it is not likely to cross couple, by absorbing or trapping it or by diffusing the light to minimize the effect of cross coupling.
An integrated optics chip, according to the present invention comprises a substrate formed of an electrooptically active material with an optical waveguide network being formed on a first surface of the substrate. The optical waveguide network has an input facet where optical signals may be input to the optical waveguide network and an output facet where optical signals may be output from the optical waveguide network. The integrated optics chip according to the present invention further includes a structure located in an upper layer of the substrate arranged to prevent surface waves that propagate in the substrate from coupling into the output facet.
The structure that prevents surface waves that propagate in the substrate from coupling into the output facet may comprise a first layer that includes a first metal and a second layer that comprises a second metal. The first layer preferably comprises titanium, and the second layer preferably comprises gold.
The structure that prevents surface waves that propagate in the substrate from coupling into the output facet may comprise a region of the substrate that has been processed to have a second refractive index that differs from the refractive index of the remainder of the substrate.
The structure having the second refractive index may be formed as a focusing region having a focal length that directs surface waves away from the facets of the optical waveguide network, or diffuses the surface propagating rays to minimize cross coupling.